1. Field of the Invention
The present invention relates to a temperature measuring technique of a high melting point metal carbide—carbon system material thermocouple type, and method for producing the apparatus, and in more detail, relates to the temperature measuring technique suitable for measuring a high temperature exceeding 2000° C., and particularly, relates to a temperature measuring apparatus capable of continuously carrying out temperature measurement from a room temperature to a maximum temperature in an apparatus having a vertical type electric furnace construction as represented by a hot isostatic (HIP) apparatus.
2. Description of the Related Art
In the high temperature process being utilized industrially, the success or failure of the process greatly depends on the reliability of accuracy of temperature measurement, reproducibility and the like. As the industrial temperature measuring technique, a thermocouple has been often used recently in view of accuracy of temperature measurement and simplicity. However, at present, in the high temperature range exceeding 2000° C., the mere usable thermocouple is limited to a tungsten—rhenium (W—Re) system thermocouple. However, in the commercial production of the W—Re thermocouple, those other than element wires of thermocouple whose diameter is 0.5 mm are not general, and due to the fact that a wire drawing method is used in producing the fine-diameter thermocouple element wires, when the element wires are used in the high temperature range in excess of 1600° C., they tend to be broken easily due to the embrittlement phenomenon resulting from higher-degree of crystal grain growth of the thermocouple element wires to shorten the service life, thus posing a problem that the breakage thereof often occurs in the midst of actual use. Further, there is a basic problem in that at high temperatures substantially in excess of 2300° C., the temperature measurement cannot be made by the W—Re system thermocouple.
For overcoming the problem of the breakage resulting from the fine-diameter thermocouple element wires according to the wire drawing method of the W—Re system thermocouple, there has been proposed, (see Japanese Patent Publication No. 17075/1991 Publication), a W—Re system thermocouple employing the construction in which using a large-diameter (3 to 5 mm) rod-like tungsten (W) and tungsten-rhenium (W—Re) system materials, one ends of these two rods are connected by the thread construction using a button-like connecting member formed of any metal, and the entirety is encased into a sheath formed of a heat insulating material such as a high melting point metal or boron nitride (BN), which is used as the thermocouple for HIP apparatus capable of carrying out operation in the high temperature range up to approximately 2200° C. It is proved that this thermocouple can be used in the stabilized manner for a period of time to a degree not industrially posing a problem in the temperature range up to 2200° C., which is being used mainly for the HIP process of silicone nitride ceramics. However, in the actual circumstances, at the high temperature exceeding 2200° C., there still remains a problem in terms of stability in case of a long-term use.
On the other hand, a thermocouple having a boron carbide sintered body rod and a pipe with a graphite bottom connected by insert construction at upper ends thereof has been sold by ESK (Electro Schmelzwerk Kempten) Ltd. This thermocouple is also able to make temperature measurement from a room temperature to a considerably high temperature range, but leaves a problem of stability of thermal electromotive force and a problem of being insufficient for continuous temperature measurement at 2300° C. since the upper limit of using temperature is less than 2300° C. at the most. In the thermocouple comprising a combination of the boron carbide sintered body and graphite, there has been experienced that the boron carbide sintered body is used as electrode material, but since density of the sintered body is not 100%, for example, when used for high temperature measurement in excess of 2000° C. in the HIP apparatus, the sintering further progresses to shorten the length of a boron carbide sintered body rod material. This is caused by the fact that since most of high-purity carbides has a high melting point, and despite high-density sintering cannot be done by a normal sintering method unless a sintering assistant is added, the use of the sintering assistant is not allowed because of being greatly affected by the use of an additive such as a sintering assistant, for securing stability of thermal electromotive force.
In view of the aforementioned circumstances, in the actual circumstances, in the apparatus used for the processing process in a temperature range exceeding 2000° C., for example, such as a high temperature sintering furnace, a hot press apparatus and part of HIP apparatus, an optical temperature measuring technique such as a two-color thermometer making use of radiant light is often used. However, in the case of the optical temperature measurement technique, the sufficient temperature accuracy is not obtained unless temperature is high in excess of 1000 to 1100° C. at which intensity of the radiant light becomes high, posing a problem that it cannot be used unless the temperature is substantially above such a temperature range as described. Consequently, in fact, there has been taken a procedure that at a temperature below the above temperature range, a temperature measurement apparatus capable of measuring is used, and for example, in case of an electric furnace, power is put according to a predetermined program, and in a temperature range in excess of 1000 to 1100° C. at which output of an optical temperature measuring apparatus is detected, an input of a temperature control device is switched to the optical temperature measuring apparatus to control temperatures, which procedure is very hard to use in the actual circumstances.
As mentioned above, there has been known, as the temperature measuring technique in the high temperature process being utilized industrially, a W—Re thermocouple using a rod-like electrode, a thermocouple having a boron carbide sintered body and graphite combined or an optical temperature measuring apparatus. However, in the W—Re thermocouple or the thermocouple having a boron carbide sintered body and graphite combined, it is possible to measure temperature up to a high temperature of approximately 2000° C., but when exceeding 2300° C. above 2000° C., the stabilized temperature measurement cannot be expected. In addition, in the case of the later thermocouple having a boron carbide sintered body and graphite combined, there is a possible problem that when it is applied to a processing apparatus to which pressure is applied as in the HIP apparatus, sintering further progresses to shorten the length of the boron carbide sintered rod. On the other hand, in the optical temperature measuring apparatus, the sufficient temperature accuracy is obtained at a high temperature of 1000 to 1100° C. or above, but conversely, at 1000° C. or below, the sufficient accuracy of temperature measurement cannot be expected.
The present invention has been accomplished in order to overcome the aforementioned problems. The object provides a temperature measuring apparatus of a high melting point metal carbide—carbon system material type capable of continuously, stably and precisely measuring temperatures up to the range from a room temperature to a high temperature in excess of 2000° C. irrespective of presence or absence of pressure in order to control the high temperature process with good accuracy.